This invention relates to the ammoxidation of paraffin hydrocarbons to unsaturated nitriles, particularly alpha, beta-unsaturated nitriles.
The value of alpha, beta-unsaturated nitriles is generally well recognized with acrylonitrile being among the most valuable monomers available to the polymer industry for producing useful polymeric products. Acrylonitrile is useful in the preparation of synthetic fibers, synthetic rubbers and other useful plastic products.
Many processes, catalytic and non-catalytic, are known and practised for the manufacture of alpha, beta-unsaturated nitriles. A generally practised catalytic ammoxidation process is performed in the vapor phase in the presence of a catalyst. For the production of acrylonitrile, propylene is the olefin reactant.
Propane is a source of carbon which is lower in cost than propylene or any other material useful as a starting material in the manufacture of acrylonitrile. Therefore, it is readily recognized that a feasible process for producing acrylonitrile directly from propane would be highly desirable.
Although some art has developed on the ammoxidation of propane to form acrylonitrile, a commercially feasible process has not heretofore been reported becaue the because yield of acrylonitrile obtained from propane is relatively low. For example, U.S. Pat. No. 3,365,482 discloses the use of molybdenum oxide or tungsten oxide as catalysts for the conversion of propane to acrylonitrile. However, it is observed from this reference that the ultimate yield of acrylonitrile, based on propane converted, is low.
A process applied to the ammoxidation of propylene or isobutylene to acrylonitrile or methacrylonitrile respectively is described in U.S. Pat. No. 3,478,082. This process comprise heating a fluidized ammoxidation catalyst in the presence of a gas comprising molecular oxygen to yield, in a first or oxygenation zone, the catalyst in oxygenated form transporting the oxygenated catalyst therefrom by fluidized solids transportation techniques to a second or reaction zone contacting the oxygenated catalyst in a fluidized state with an unsaturated hydrocarbon and ammonia in the second or reaction zone to yield the nitrile product, and recycling the oxygen-depleted catalyst back to the first, or oxygenation zone to complete the cycle and reoxygenate the catalyst. This type of operation is hereinafter referred to as a transported bed reaction. However, contrary to the disclosure in that specification, it has now been found that transported bed reaction processes can be applied to the ammoxidation of saturated hydrocarbons of the paraffin series to yield alpha, beta-unsaturated nitriles providing a suitable catalyst is used.
In U.S. Pat. No. 839,398 a modified form of transported bed reactor is used to form V.sub.2 O.sub.4 which is then used to ammoxidize various hydrocarbons (though paraffin hydrocarbons are one of the few group of hydrocarbons not specified). The catalyst used is V.sub.2 O.sub.4 as opposed to the V.sub.2 O.sub.5 of the prior art and this is the nub of the invention since it is said to reduce by-product incidence.
Another reactor in which the catalyst is cycled is disclosed in U.S. Pat. No. 3,639,103 but the cycling is in effect between stratified zones in the same reactor and such a configuration confers few of the advantages of a genuine transported bed reactor such as is used in the present invention. Moreover, the invention does not address itself to the problem of catalyst stability under oxidation/reduction cycle conditions nor to the ammoxidation of paraffin hydrocarbons but is confined to disclosure of a novel type of fluidized bed (as opposed to transported bed)reactor.
Moreover, not all catalysts which are effective in the ammoxidation of hydrocarbons to nitriles can be used in transported bed reactors since the nature of the reactor demands that they be stable to repeated oxidation/reduction cycles at elevated reaction temperatures. Thus, a suitable catalyst is one which is capable of efficiently ammoxidizing saturated hydrocarbons and which is capable of operating under repeated oxidation/reduction cycle conditions at elevated temperatures. One solution to this problem is set forth in U.S. Pat. No. 3,652,638 in which instead of a fluidized catalyst bed, a catalyst is used which comprises a molten metal oxyhalide which gives up its oxygen and is in a separate zone reconverted to the oxyhalide. This does not, of course, utilize the principles of a fluidized bed reactor or a transported bed reactor but it does illustrate an alternative solution to the problem of finding a catalyst stable under reaction conditions which comprise repeated oxidation/reduction cycles at high temperatures.
A suitable catalyst has now been found that can be used in conjunction with transported catalyst bed technology to produce unsaturated nitriles from paraffinic hydrocarbons more efficiently than by the hitherto-known processes.
The present invention has as its objects, the provision of an improved vapor phase process for the production of alpha, beta-unsaturated nitriles by ammoxidation of paraffin hydrocarbons. A further object is to provide a transported catalyst bed process for the ammoxidation of paraffin hydrocarbons to alpha, beta-unsaturated nitriles and more specifically for the conversion of propane and isobutane to acrylonitrile and methacrylonitrile respectively.
Other objects and advantages will become apparent to the skilled reader upon study of the disclosures contained herein and the appended claims.
The present invention provides a process for the ammoxidation of a paraffin hydrocarbon to an unsaturated nitrile which comprises reacting a paraffin hydrocarbon with ammonia and oxygen at a temperature of from 350.degree. to 550.degree. C in the presence of a catalyst having the empirical formula Sb.sub.a U.sub.b Fe.sub.c W.sub.d O.sub.e wherein a is 1 to 10, b is 0.01 to 1, c is 0 to 1, d is 0 to 0.1 and e is a number chosen to satisfy the valencies of the other elements in the oxygenation states in which they appear characterized in that the catalyst has previously been oxygenated by heating in a molecular-oxygen containing gas, and the oxygen required by the ammoxidation reaction is provided entirely by the oxygenated catalyst.
The process of the invention is particularly useful when operated in conjunction with a transported bed reactor and the following description will be related to such a reactor for greater clarity.
Thus a specific preferred embodiment of the present invention comprises a process for the ammoxidation of a paraffin hydrocarbon to an unsaturated nitrile which comprises:
a. passing a molecular oxygen-containing gas through a fluidized bed of a catalyst in an oxygenation zone, said catalyst having the empirical formula Sb.sub.a U.sub.b Fe.sub.c W.sub.d O.sub.e, wherein a is 1 to 10, bis 0.01 to 1, c is 0 to 1, d is 0 to 0.1 and e is a number chosen to satisfy the valencies of the other elements in the oxidation states in which they appear, so as to oxygenate the catalyst;
b. continuously forwarding oxygenated catalyst in a fluidized state to a reaction zone and, in said reaction zone, contacting said oxygenated catalyst in the fluidized state with a paraffin hydrocarbon and ammonia such that the paraffin hydrocarbon and ammonia react with oxygen from the oxygenated catalyst, at a temperature of from 350.degree. to 550.degree. C to produce the unsaturated nitrile, the oxygenated catalyst providing the only source of oxygen introduced into the reaction zone; and
c. continuously removing catalyst from which oxygen has been removed from the reaction zone and transporting it in a fluidized state to the oxygenation zone.
The process of the invention is particularly effective in the production of acrylonitrile and methacrylonitrile from propane and isobutane respectively.
This type of process has several advantages relative to other ammoxidation processes for making nitriles. A major advantage is that it allows higher selectivities to be realized. Since no gas phase oxygen is added to the ammoxidation zone, gas phase side reactions with oxygen are minimized and catalytic reactions are favored. Also, higher selectivities are realized since the solids are not circulating within the reaction zone as in a fluid bed but moving more closely to plug flow as they pass through the reaction zone and backmixing of the gas is therefore minimized.
Another advantage is that the reaction products are much more concentrated than with the conventional process. Since the oxygen is fed to the reaction zone as part of the circulating solid catalyst instead of as air, the nitrogen that comprises the major portion of the air is not present to dilute the reaction zone off gases. This greatly decreases the size of units required to achieve separation of product from unreacted gases and makes simpler processing possible.
A third advantage is the safety of this system. There are no places in the apparatus where explosive mixtures of hydrocarbons and oxygen are present. Thus, much higher concentrations of hydrocarbons can be safely utilized than is the case where the paraffin is contacted with the catalyst in admixture with oxygen.
Yet another advantage lies in the option of using halogen promoters in the substantial absence of gas phase oxygen thus reducing the incidence of competing reactions affecting the activity of the promoter and potentially liberating corrosive halogen compounds.
The catalyst used in the process of the invention is one that can exist in an oxygenated form and is stable to repeated cycles in which oxygen is added and removed from the catalyst. While the catalyst forms have been characterized as "oxygenated" and "oxygen-depleted" it is not to be assumed that the oxygen is necessarily present in chemically combined form. While this is not excluded the major part of the oxygen taken up is usually adsorbed on the catalyst surface or even trapped within the catalyst structure. All such forms are understood to be embraced by the term "oxygenated" as used herein.
The active components of the catalyst are antimony and uranium and while these usually exist in the form of separate simple oxides, it is possible to have them present as complex oxides. The catalyst may contain other components which modify the activity of the catalyst such as iron and tungsten. In many cases the presence of iron and tungsten is desirable since, as is demonstrated in the Examples, replacing part of the uranium by iron and tungsten results in increased selectivity to the desired acrylonitrile product. These additional components too may be in the form of separate oxides or in the form of complex compounds with one or more of the other components and oxygen.
These include the catalysts described in U.S. Pat. Nos. 3,198,750 and 3,886,096.
Catalysts useful in the present invention have the empirical formula Sb.sub.1.sub.-10 U.sub.0.01.sub.-1 Fe.sub.0.sub.-1 W.sub.0.sub.-0.1 O.sub.y wherein y is the number of oxygen atoms required to satisfy the valency states of the antimony, uranium, iron and tungsten.
The catalyst can be employed without support, and will display excellent activity. It is usual however in any fluidized catalyst bed that the active components be combined with a support, and preferably at least 5% up to about 90% and more preferably 5 to 50%, of the supporting compound by weight of the entire composition is employed in this event. Any known support materials can be used, such as, for example, silica, alumina, zirconia, alundum, silicon carbide, alumina-silica, and the inorganic phosphates, silicates, aluminates, borates and carbonates stable under the reaction conditions to be encountered in the use of the catalyst.
The ammoxidation reaction can conveniently be conducted at temperatures of, for example, 350.degree. to 550.degree. C but the preferred reaction temperatures are in the range of 450.degree. to 525.degree. C and most suitably 470.degree. to 510.degree. C.
The reaction is preferably conducted at substantially atmospheric pressure though higher pressures can be used if desired. However, since the reaction proceeds satisfactorily at atmospheric pressures the advantages of working at higher pressures are not generally such as to justify the expense of the high pressure equipment involved.
Since the major factor influencing the rate of conversion of the paraffin to nitrile is the amount of oxygen available from the catalyst, the conversion rate is largely independent of the partial pressures of the ammonia and paraffin fed to the reaction zone. It can be appreciated, therefore, that the rate-determining factor is the rate at which the catalyst can be recycled to the reaction zone after oxygenation. The provision of substantially all the oxygen required by the reaction through the oxygenated catalyst is not a disadvantage in that it results in a higher selectivity towards the desired nitrile product than would be the case if the available oxygen were in greater supply as is the case in processes where a gaseous hydrocarbon/oxygen mixture is reacted. Generally, the amount of available oxygen carried by the oxygenated catalyst is up to 1% and more generally from 0.1 to 0.8% and preferably 0.2 to 0.6% by weight of the active catalytic components of the catalyst.
While, as indicated above, the partial pressures of paraffin and ammonia are not rate-determining factors, it is generally preferred that the reactants be present in proportions that differ by less than 20% and preferably less than 5% by volume.
The catalyst is oxygenated or reoxygenated by heating it in the presence of a regenerator gas comprising molecular oxygen. This can be pure oxygen but more usually it is air or a mixture of gases comprising oxygen and other inert gases such as nitrogen, helium or neon.
The temperature at which the catalyst is oxygenated is conveniently that at which the ammoxidation reaction is conducted but higher or lower temperatures within the preferred reaction zone temperatures can be used if desired.
In general, it is found preferable to arrange that the catalyst residence time in the oxygenation zone be longer than that in the reaction zone perhaps by a factor of two or more such as for example 2 to 3. This can easily be achieved by, for example, adjusting the relative lengths of the zones where these zones have identical cross-sections.
The paraffin hydrocarbons to which the process of the invention most suitably applies are those comprising 3 to 5 carbon atoms such as propane, butane, isobutane, pentane and iso-pentane. The main utility of the invention, however, lies in the production of acrylonitrile and methacrylonitrile from propane and isobutane respectively.
A halogen promoter may be employed in the process of this invention. The halogen can be introduced into the reaction in any suitable manner. For example, the halogen may be introduced along with the paraffin hydrocarbon and ammonia as elemental halogen or, better, as a volatile halogen-containing compound. Alternatively, but less preferably, the catalyst can be treated in the oxygenation zone with the halogen promoter. Any halogen can be used, but at the present, bromine is the preferred halogen. Suitable volatile halogen-containing compounds are the halo-alkanes having up to 3 carbon atoms, Examples are CH.sub.3 Br, CH.sub.3 Cl, CH.sub.3 I, CH.sub.3 F, CH.sub.2 Br.sub.2, CH.sub.2 Cl.sub.2, CHF.sub.3, CHI.sub.3, CBr.sub.4, CCl.sub.4, C.sub.2 H.sub.5 Cl, C.sub.2 H.sub.5 F, C.sub.2 H.sub.4 Br.sub.2, C.sub.2 H.sub.4 I.sub.2, C.sub.2 H.sub.3 Br.sub.3, C.sub.2 H.sub.3 I.sub.3, C.sub.2 H.sub.2 Br.sub.4, C.sub.2 HCl.sub.5, C.sub.2 Br.sub.6, C.sub.3 H.sub.7 I, C.sub.3 H.sub.7 Br, C.sub.3 H.sub.6 Cl.sub.2, C.sub.3 H.sub.5 Br.sub.3, C.sub.3 H.sub.5 Cl.sub.3, C.sub.3 H.sub.2 Br.sub.6, C.sub.3 HCl.sub.7 and the like. The ammonium and hydrogen halides such as ammonium bromide, chloride and iodide and hydrogen fluoride, chloride and bromide can also be used with advantage. Elemental bromine is a further available option. When treating the catalyst with the halogen, a metal halide, such as the halides of lead, iron, aluminum, zinc and the like, or a non-metal halide such as an ammonium halide can be used. Generally speaking, regardless of the means of introducing the halogen promoter, the promoter will be employed in a mole ratio of from 0.00005 to 0.10 and preferably from 0.005 to 0.05 mole of halogen (measured as X.sub.2 where X is flourine, chlorine, bromine or iodine) per mole of hydrocarbon used.